Cassini continues making loop-de-loops around Saturn, returning tens of thousands of way cool pictures. Like this one:

From 1.3 million kilometers (800,000 miles) away — 3 times as far as the Moon is from the Earth — Cassini spied this pretty scene. It shows, obviously, Saturn’s rings to the right. The very thin ring extending to the left is the F-ring; it’s very faint and wasn’t even discovered until 1979, when Pioneer 11 passed the planet.

The two moons are Pandora (the flying saucer-shaped one) on the left, and Epimetheus on the right. Usually, in pictures like this, perspective is a problem; one moon is much farther away than the other, so your sense of scale gets a bit bollixed. But in this case, both moons are about the same distance from Cassini! Pandora is about 114 x 84 x 63 km (68 x 50 x 38 miles) in size, and Epimetheus is a bit heftier at about 144 x 108 x 98 km (86 x 64 x 58 miles). In this shot, the rings are in the background relative to the moons, and Pandora is just a hair closer to Cassini than Epimetheus.
I was surprised that they appeared so close together, so I did some checking. Pandora orbits Saturn at a distance of about 141,700 km (85,000 miles), and Epimetheus orbits at 151,400 km (91,000 miles). So really, they never get any closer than 10,000 km (6000 miles) to each other. Since they do look pretty cozy in this image, there really is a little perspective going on, since Epimetheus must be a few thousand kilometers farther away. That’s only a trifle compared to the more than 1 million km distance Cassini was from the pair when it took this shot, though. What this means is that if you compare the sizes of the two moons you get a good idea of their relative diameters, but their positions relative to Saturn are a little messed up due to perspective. Got it?

Once I got the orbital distance of the moons, I was curious how long it takes them to orbit Saturn. Turns out, it only takes about 15 hours for Pandora and about 17 hours for Epimetheus! So both moons are screaming around the planet at speeds of roughly 60,000 kph (36,000 mph), far faster than even low Earth satellites move. That’s because Saturn is a lot more massive than the Earth, and has far more gravity. It yanks much harder on those moons, whipping them around at greater speeds.

There’s more, too. Pandora is one of the shepherd moons of the F-ring, helping it maintain its shape. It shares an orbit (more or less) with the moon Prometheus. As it happens, Epimetheus shares an orbit (more or less) with the moon Janus.

Now follow along here: in mythology, Epimetheus and Prometheus were very close brothers. Their names means hindsight and foresight, respectively. Prometheus gave us fire and civilization, and had his liver pecked out by birds every night for his sins against the gods. Epimetheus was supposed to give mankind positive traits, but was a bit of an absent-minded goofball, and he ran out of raw materials before he got to us. For this, the gods gave him the "gift" of Pandora, whom he married.

Well, that’s not fair! Being smart and clever and helpful gets your organs ripped out of you, and being an idiot with no eye for the future gets you rewarded*.

Of course, in reality, Epimetheus’s wife shares an orbit with his brother. That’s gotta hurt.

Sigh. I think I prefer the confusion of the actual Prometheus and Epimetheus to the confusion of their mythical namesakes. Reality may not always be fair, but at least (to borrow a phrase from George Hrab) it’s fair in its unfairness.

* Feel free to extrapolate this myth to science and politics, if you wish.

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Actually, Pandora was a punishment for Prometheus and humanity for the stolen fire. Prometheus, been as awesome as he was says “bro, thnks but no LOL” so the impulsive Epimethius as impulsive as it was says “free hot chick, zink!” and go all the way marrying her… well we all know how that ended (sorry for the random chatter, currently reading Aeschylus’ Prometheus Bound and kind hype about it)

Well, tricky topic here, Phil, but the surface gravity of Saturn is only about 90% the surface gravity of Earth due to its very low density. Of course, that is at a substantially larger distance from the center of mass than you would get if you were on Earth! You are correct that the force of gravity at the orbital distance of the Moons is much greater than it would be at the same orbital distance on Earth, but most people think about the surface gravity of a planet when you say one has more gravity than another. Just a subtle distinction for you.

Kind of related question, and something I’ve been wondering for a long time: how is the rotational period for the gas giants determined? I mean, they all have high winds, storms and such, but I assume that the rotational period is not determined by the cloud movements, right? Any thoughts?

but I assume that the rotational period is not determined by the cloud movements, right? Any thoughts?

I’m not sure & I could well be wrong – please correct me if I am – but I think cloud patterns *are* actually used along with perhaps magnetic fields to judge the gas giants rotation periods.

Its a good question and pretty hard to work out given these planets are gas and fluid rather than solid bodies.

I vaguely recall reading somewhere that Saturn’s rotation period has actually changed between the time Voyager II last flew past the planet in 1980-83 ish (?) to when Cassini arrived in 2004 ~ish (?) – although maybe it was just a case of a more precise measurement being made. I’ll have to research this further myself now.

Surface gravity: I looked up Saturn’s on the web the other day and the first seven sites, all reasonably good educational or academic sources, gave me seven different values ranging from 1.12 to 0.87 times that of Earth. They were all using different definitions of ‘surface’. One was ‘the level at which Saturn’s atmospheric pressure equals that of Earth’ as though that meant anything.

If you look carefully, there’s four, maybe five glitches in the middle of the three outermost rings – but are they all caused by the two moons, especially the one closest to the viewer just before the three rings appear to merge?

If you look at the moons themselves, both of them seem to have highly visible craters on their upper bodies, but Epimetheus, (and Pandora, to a degree) appears to have a highly excavated – impact created? – ‘under’ region, adjacent to which is what almost appears to be a mini-me moon, (although viewed in inverted colours it could be a pyramid-esque mountain, but surely that’d be almost as odd?).